Yeasts have been marketed in the form of moist cultures or compressed cakes because it has been found that improper dehydration or even drying outside of very narrow limits can cause the yeast to suffer a shock and thereby lose some or all of its life force or vitality. In order to prevent such shocks, until now, it has been the practice to dry the yeasts at relatively low temperatures and at relatively high humidity. This required the use of rack and tray driers or continuous belt driers and required long drying times in order to keep the moist yeast alive until sufficiently dehydrated to reach the dormant stage at which there is a suspension of the yeast metabolism. Such dormant yeast can be regenerated.
Apart from requiring complex and expensive operating and control apparatus, slow drying also has the disadvantage of reducing the vitality of the yeast. The yeast, while still moist, during the lengthy drying procedure, lacking other sources of nutriment, metabolizes its stored nutriments as well as its own substance. Upon rehydration, the regenerated yeasts, are found to have lost a substantial portion of the original vigor and vitality.
The terms, vigor and vitality, as used herein, refer to the qualities of growth and reproduction, respectively, of the yeast.
Some improvement has been attained by dehydrating the yeast in two drying stages: a first stage, preliminary drying in moist air (relative humidity of 75 to 80%) and, thereafter, a final drying stage with dry air. This process is described in German Pat. No. 857 784 and is energy inefficient, requires inordinately long drying times and expensive apparatus.
Also disclosed in German Pat. No. 857 784 is an apparatus by which moist yeast can be granulated before drying. The granules are there formed by forcing the moist yeast through an apertured metal plate and subdividing the resultant strands into short sections by cutting elements moving behind the apertured plate. Since the cutting elements have relatively smooth surfaces and sharp cutting edges, provision is made in this prior art for rounding off the resulting strand sections by rolling them in a granulating drum before or during the initial drying stage. Since the resulting rolled yeast granules are only surface treated, the drying still requires the aforementioned two-stage process of a preliminary moist-air drying and the subsequent dry-air drying.
U.S. Pat. No. 2,921,854 describes a method of aggregating powdered yeast having sizes from 5 to 20 microns to give particles substantially retained on a 60-mesh sieve. An agglomerated powdered dry yeast having a diameter of a little greater than 0.25 mm is obtained.
U.S. Pat. No. 3,843,800 describes a process for drying compressed yeast granules where the yeast cells are held within a temperature range of 20.degree. C. to 50.degree. C. in a fluidized bed type of dryer at atmospheric pressure. This compressed yeast is extruded to form strands. After mixing the dough for 6 minutes at 28.degree. C., complete rehydration occurs.
DE-OS No. 26 37 601 describes a process for drying of an extrudate granule from a low pressure extruder where after drying to a solids content of about 70%, the semi-dried aggregate is ground to give a powder which is then further dried.
U.S. Pat. No. 2,710,810 relates to a process of vacuum dehydration of microbial liquid cultures. Under these conditions, the concentrate expands and the final product is also in an expanded state. This expansion is caused by the entrapment of a multitude of small steam bubbles throughout the mass as a foaming operation. Fast rehydration occurs by this process. It is suggested that this expansion can be enhanced by introduction of an inert gas into the liquid concentrate. The example employs a liquid concentrate containing 25% solids to which is added corn syrup solids to give a 50% solids concentrate. This concentrate was vacuum dehydrated at a temperature of from 200.degree. F. to 80.degree. F. and expanded 16 times.
Before use, the dried yeast granules must be rehydrated by moisture in order to regenerate the now passive or dormant yeast cells. During this regeneration by rehydration, the individual yeast cells absorb water, and regain their normal appearance. This regeneration and moisture reabsorption, however, takes place over a finite time period. During the initial phases of the regeneration period there is the danger of malabsorption leading to "bleeding" or exudation of the cell contents of some of the cells thus damaging or even killing the entire yeast culture. To minimize this exudation phenomena caused by the rupture of the cell walls, it is usual to rehydrate the dried yeast granules by suspending them in water at temperatures in the range of 30.degree. to 40.degree. C. Within the stated temperature range, the hydration and regeneration take place with a minimum malabsorption, cell wall disruption, and a prevention of the reduction of the vitality of the yeast occurs by exposure to too high a temperature.
During the rehydration of the dried yeasts, granulated according to the previous procedures, difficulties have been noted such as uneven regeneration of yeast cells in each granule. Cells on the exposed outer surface of the granules rehydrate and swell and thus hinder the penetration of the moistures to the dormant yeast cells at the interior portions of the granule. Accordingly, it is noted that the dried yeast granules of the prior art take a long time to regenerate and, further, because of this time span, the danger of ruptured cell walls is increased with its resulting losses in vitality and vigor of the regenerated yeast.